Thera Macula (highlighted in false color)
is an area of "chaos terrain" on Europa
which may overlay a subsurface lake within
the moon's outer shell of ice
(more).

On November 16, 2011, scientists announced that data from NASA's
Galileo probe
(which operated from 1989 to 2003) appear to reveal at least two
bodies of liquid water the volume of the North America's
Great Lakes
underneath the surface ice of Europa. Some scientists believe
that such subsurface lakes of water provide potential habitats
for Earth-type life. Galileo images of "two roughly circular,
bumpy features" on Europa's surface ("chaos terrains") are
similar to features on Earth
involving ice shelves or glaciers overlaying volcanoes where
thermal energy is transferred to the surface. Chaos features
on Europa's surface are likely to be formed by mechanisms that
involve significant heat exchange between the icy shell and the
underlying lake, which may provide a pathway for transferring
nutrients as well as energy between the surface and the vast
global ocean thought to exist below the thick ice shell (NASA
science
news; and
press
release).

Lakes underlying areas of "chaos terrain"
on Europa's shell of ice may provide
suitable habitats for Earth-type life
(more).

Conditions Condusive for Life?

Europa (which is 12 percent smaller than Earth's Moon) appears to have
a sparsely cratered shell of water ice that may be only 30 to 50 million
years old, and so some resurfacing process must be renewing its icy
shell. Most important, some of the ice below appears to have been
melted by tidal heating from gravitational inteeractions of its mantle
and core from its eccentric orbit about Jupiter and radioactive decay to
create a deep global ocean of electrically conductive liquid such as
salty water that generates fluctuations in the moon's magnetic pole
(Walter
S. Kiefer, 2002). Although
past
detailed mapping and measurements of impact craters on Jupiter’s large
icy moons suggested that the thickness of Europa's ice crust ranges
from around 12 miles (20 km) to 16 miles (25 km) thick, researchers
hope that refined methods using combined measurements of gravity and
the magnetic field made from orbit can better determine the ice crust's
thickness as well as its salinity. by observing how the moon flexes and
deforms and by measuring magnetic variations (Washington University
press
release). Although NASA terminated funding in 2005 for a proposed
Jupiter
Icy Moons Orbiter (which would have searched for evidence of sub-ice,
oceanic life on Europa, Callisto, and Ganymede), the Agency was still
funding the development of a robotic submarine for exploring the
sub-ice oceans of those icy moons in 2007
(Kathleen
M. Wong, New Scientist, December 14, 2007).

Europa is covered by a shell of ice
with colorful fractures that may overlay
a deep global ocean
(more).

Europa's ocean is believed to be around 62 miles (100 km) deep, ranging
mostly between 80 and 170 km (50 and 110 miles. Such an ocean would
have a volume equivalent to all of Earth's oceans combined. Even deeper
oceans of 60 to 125 miles (100 to 200 km) are now believed to lie below
surface ice on Callisto and Ganymede, which orbit further out from
Jupiter. (More discussion of the ice-thickness issue is available at
the Lunar
and Planetary Institute).

Europa is now thought
to have an global ocean
of salty water or slush
rather than warm
convecting ice below
its icy crust
(more).

NASA's
Galileo
mission detected both carbon dioxide and sulphur dioxide, a possible
sign of volcanism, leaking from certain areas on Europa's surface
(Kathleen
M. Wong, New Scientist, December 14, 2007).
As significant uncertainties about the thickness of the surface ice still
exist, some planetary scientists have identified two possible mechanisms
for how possible volcanic heat can escape to the surface from Europa’s
rocky mantle and be carried upward by buoyant oceanic currents. If the
subsea heat is intense and the ice shell is thin enough, the surface ice
shell can melt directly and cause regions of what appear to be broken,
rotated, and tilted ice blocks, called
"chaos"
terrain
(Nimmo
and Giese, 2004;
Spaun
et al, 1999 and
1998;
and
Greenberg
et al, 1998).
On the other hand, if the ice shell is sufficiently thick, the less intense
interior heat can be transferred to warmer ice at the bottom of the shell,
with additional heat generated by tidal flexing of the warmer ice which can
slowly rise and flow as do glaciers do on Earth; this slow but steady motion
may also disrupt the extremely cold, brittle ice at the surface to produce
the chaos regions. As tides raised by Jupiter in Europa’s ocean rise and
fall, they may cause cracking, additional heating, and even venting of water
vapor into the airless sky above Europa’s icy surface.

The thickness of Europa's
icy crust is still uncertain
without additional data on
the amount of heat available
from subsea volcanism
(more).

Potential Habitats

Biologists have speculated that, if liquid water does exist beneath
Europa's shell of ice, then Earth-type microbial and even larger
lifeforms may have developed. The near-infrared mapping spectrometer on
NASA's Galileo spacecraft found evidence of the presence of molecules
made of oxygen, carbon, sulfur, hydrogen, and nitrogen on Europa, and
a hint of the presence of a class of complex organic compounds called
tholins which may be driven by pre-biotic processes
(more).
Europa's briny ocean may also have an acidity similar to battery acid,
rich in the sulfur compounds found on the other large Jovian moons.
The presence of sulfur and sulfuric acid, however, may have also been
present on the early Earth and is compatible with many extreme
environments where bacteria have evolved biochemical processes to
extract energy without sunlight (e.g., to extract energy from the
charged reactions of sulfuric acid).

Hydrothermal vents on the
ocean floor known as "black
smokers" may support
undersea habitats on Europa,
as on Earth
(more).

Potential Europan habitats include deep-sea colonies based on
heat-loving bacteria like those found around
hydrothermal
vents on Earth's ocean floor. These vents (also know as black
smokers) expel minerals dissolved by water which bacteria use to
in chemical reactions to produce energy and support ecosystems
involving larger multi-cellular lifeforms that feed off of of the
massive and fast-growing bacterial populations. Biologists
believe that life on Earth could have started in such hot habitats
sulfurous around hydrothermal vents, because the vent environment
have little oxygen and harmful Solar radiation which can damage
primitive molecules. Many organic molecules needed for life could
even have formed below the ocean floor from interactions between
minerals and circulating hot water driven these hydrothermal ven
systems.

Photosynthetic life might
support "crack" habitats
in thin ice, if daily tides
force water into the cracks
formed by gravitational
flexing from Jupiter
(more).

In 2002, planetary scientist
Richard
Greenberg proposed that Europa's biosphere could include
lifeforms adapted to niches provided by the cracks in its icy crust.
Although radiation from Jupiter's magnetosphere would be harmful to
life on Europa's surface down to a depth of a few centimeters,
sunlight could sustain photosynthetic organisms beneath the ice to
a depth of several meters. Clinging lifeforms might scale the walls
of the cracks and even hibernate within the walls, whereas floating
life forms may be able to move with the tides as the cracks open and
close on a daily cycle
(David
L. Chandler, New Scientist, October 20, 2002). If past
analysis
of Europa's craters with those on Callisto and Ganymede indicated,
however, that Europa's crust may relatively thick at around 12 miles
(20 km) to 16 miles (25 km) thick prove true, then such ice-crack
habitats would be unlikely.